This invention relates to electrical strain gauges.
In the design and use of electrical strain gauges, it is important to ensure that a stable response can be obtained from the strain-responsive electrical element that is directly or indirectly attached to the member the strain of which is to be measured, if accurate measurements are to be made.
For this purpose, strain-responsive elements deposited on a suitable substrate by thin-film techniques are found to be superior to elements etched from a plated-on foil. These techniques can provide a firmly adherent layer on a substrate, and so provide a strain-responsive element that is free of problems of long-term creep and hysterisis occuring in the use of foils bonded onto the substrate by a plastics adhesive (e.g. epoxy resin), as well as avoiding the temperature limitations imposed by such adhesives.
Known thin-film techniques comprise methods of forming solid layers by condensation from the vapour phase, including vacuum deposition processes, e.g. sputtering and chemical vapour deposition. Such layers are usually deposited with a thickness of less than 2 microns, although greater thicknesses are possible, and the resultant thin-film will have characteristics typical of a discontinuous layer or of a bulk material depending upon the thickness. The term "thin-film" as used herein is intended to refer to deposits produced by thin-film techniques and capable of providing a flow path for an electrical current.
Strain gauges incorporating such strain-responsive elements typically comprise a layer of glass or other insulating material deposited on a surface of the member to be monitored, e.g. by sputtering, as an insulating layer, the underlying member commonly being metallic, and a thin-film strain-responsive layer deposited on the insulating layer and etched to form a resistive circuit element. The connection of the element to an external measurement circuit is made by wire bonds using a printed circuit board also attached to the surface of the member adjacent to the resistive element for the junction of the wire bonds with the lead-outs to the measuring circuit. However, these wire bonds are relatively fragile in use and may be prone to chemical attack.
Another aspect of strain gauge measurement lies in that it is often desirable to operate in hostile environments. It is well known to cover a strain gauge element with organic encapsulating materials which serve to protect the element from dirt and moisture, but this measure is of little use at high temperatures. Moreover, the adherence of a plated-out strain gauge element will be lost if it is subjected to temperatures above the limits for the adhesive bonding materials and substrates used to attach the element to the member being monitored, and even though a thin-film element may itself be resistant to high temperatures the printed circuit board by which it is connected to the measurement circuit is not.
As a result, when the critical parameter to be monitored is the strain at a position such as the inner wall of a container, e.g. a pressure vessel, filled with heated fluid, it may not be possible to site a strain gauge on the surface in question. The measurement must then be made indirectly from another region of the container walls, with the result that there is a loss of accuracy and of stability in the strain signals generated.
Finally it may be mentioned that a form of packaging for the solid state devices is known from U.S. Pat. No. 3,444,619 to improve the durability of the device, in which the device is attached by an adhesive such as solder to a base element in apertures of which electrical leads have been fixed in insulating glass inserts, the wire bonds of the device being connected to the exposed ends of the inserted leads, and a metal cover then being soldered or welded to the base element to enclose the solid state device and the wire bond connections to the leads. Such an arrangement if employed for a strain gauge would have little value, however, in meeting the problems outlined above.